Among many uses for internal combustion engines, such engines are often employed for powering various vehicles, either as a primary power source, or as part of a hybrid powertrain. When an internal combustion engine is used in a hybrid powertrain, such an engine is combined with one or more electric motors to power the vehicle.
In order to maximize fuel efficiency in a hybrid vehicle during deceleration, such as coast down, i.e., when the vehicle is decelerating from elevated speeds due to road interface and vehicle driveline friction, as well as due to air resistance, or during braking, it is often desirable for the engine to be shut off. However, during such maneuvers, it may also be desirable for the engine to contribute to vehicle deceleration via engine braking through engine friction and pumping of air by the engine's cylinders.
Additionally, while the hybrid vehicle is in deceleration mode, vehicle inertia may be employed to back-drive the electric motor in generator mode for recharging vehicle batteries, thereby improving efficiency further. Hybrid vehicle deceleration may also be provided via a regenerative braking system, where the engine may likewise be shut off, and the otherwise lost braking energy is similarly recaptured via the electrical motor.